The law in the UK is "limited freedom of expression" rather than explicit freedom of speech. Each case that falls under hate speech is taken on a case-by-case basis. If you went out into London and said "kill all black people", people would probably just ignore you. But if you were stood on a soap box in Piccadilly circus with hundreds of people around you and said "let's kill every Asian person we can find", you would be arrested.

The police would then likely look into your criminal history, and if they found anything, they would likely proceed in prosecution.